Frequency reducing system



Feb. 1, 1944. R, H, HERQCK 2,340,382

FREQUENCY REDUCING SYS TEM Filed Dec. 24, 1942 20' LOAD HIGH FREQ. LOAD INVENTOR. ROSWELL H. HERRICK BY w w xm Mule ATTORNEYS Patented Feb. 1, 1944 FREQUENCY REDUCING SYSTEM Roswell H. Herrick, Oak Park, 111., assignor to Automatic Electric Laboratories, Inc., a corporation of Delaware Application December 24, 1942, Serial No. 169,998

12 Claims.

The present invention relates to frequency reducing systems and, more particularly, to improvements in frequency reducing systems of the static type disclosed in Patent No. 2,088,618, Stocker, and No. 2,088,619, Stacker, granted August 3, 1937.

In systems of 'this type, a resonant circuit is provided which is tuned to a fundamental or resonant frequency lower than that of the current supply source. When such a resonant circuit is periodically shock-excited, each shock excitationproduces in the circuit an alternating current of the resonant frequency having an amplitude of decreasing value due to the internal losses of the circuit and also to the abstraction of energy by the connected load. If, however, :the periodicity of shock excitation is sufficiently high, oscillation of the circuit at the fundamental or resonant frequency is sustained. Systems of this formntilize a non-linear impedance-elei'nent in the form of a saturable core reactor connectedbetween theresonant circuit and the supply source, as the vehicle for periodically shock exciting-the resonant circuit to sustain its oscillation.

Static frequency reducing arrangements of the character briefly referred to above have par-, ticular utility in the telephone field where they are utilized in the production of a ringing voltage of low frequency, which is used in the signaling of called subscriber substations. In the usual ringing or signaling operation, a revertive or ring-back tone signal is transmitted to the calling substation for the purpose of informing the calling subscriber that the called substation is being signaled. From the standpoint of audibility, it is preferable to use a ring-back or reto provide a readily discernible audible signal when reproduced ,at the end of a long line to which the voltage is applied. It has also been found that the presence of the tone coil in the resonant circuit of a system of this character, causes a reduction in the low frequency output capabilities of the system due to the absorption of the low frequency energy by the tone coil.

It is an object of the present invention to provide an improved static frequency reducing system wherein the above-noted difficulties are obviated.

It is another object of the invention to provide an improved frequency reducing system of simple arrangement which utilizes no moving parts and includes provisions for supplying low and high frequency currents to low and high frequency loads when energized from a source of current having a frequency higher than that of the low frequency output current.

It is a further object of the invention to provide an improved static frequency reducing system, wherein provisions are made for augmenting the harmonics of the supply source frequency introduced into the output circuit of the system, both as regards the magnitude of the harmonic voltages and the frequencies of the vertive tone signal of a frequency higher than the frequency conventionally utilized for the energization of the standard telephone ringer. Accordingly it has previously been the practice to utilize an iron core tone coil in the resonant circuit of a static frequency reducing system of the character described above, this coil being designed so that its core is saturated on each half cycle of the current traversing the resonant circuit, whereby harmonics of the fundamentalfrequency are produced and supplied to the output circuit of the system for use as the revertive tone signal voltage. It has been found that the revertive tone signal voltage which may be obtained from a static frequency reducing system in this manner contains a limited number of harmonics of the fundamental frequency, is of limited magnitude, and may be insufficient harmonics supplied to the output circuit of the system.

According to another object of the invention, the increase in the harmonic content of the output voltage is realized without lowering or otherwise afiecting the low frequency load carrying capabilities of the system.

It is still another and more specific object of the invention to provide a static frequency re-,

ducing system of the character described, wherein a channel separate from that used to deliver the low frequency energy to'the output circuit, is provided for transmitting the high frequency harmonic energy to the output circuit.

Theinvention, both as to its organization and method of operation, together with further obiects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawing in which the single figure illustrates a static frequency reducing arrangement connected and arranged in accordance with the present invention.

Referring now more particularly to the drawing, the static frequency reducing system there illustrated comprises a tuned resonant or oscillatory circuit which serially includes a condenser II, the primary winding l5 of a transformer l3 and the winding of a start relay 2!. This resonant circuit is connected through the winding 01' a saturable core reactor l I, a protective device in the form or a fuse 24, and the contacts of a circuit breaker 25 to the terminals of a source of alternating current III of low internal 11 npedance which may comprise any suitable alternating current generator, transformer, power line, or network, capable of supplying the necessary power at the frequency required to operate the system. In the illustrated arrangement of the system, an output circuit comprising the secondary winding I 4 of the transformer I3, is provided for delivering twenty cycle and high frequency currents'to the loads I8 and I9, respectively. A condenser 22 of the proper size to block the passage of low frequency twenty cycle currents, is used for transmitting the high frequency components of the output current to the high frequency load l9. For the purpose of adjusting the magnitude of the low frequency voltage applied to the input side of the load IS, a twoposition switch 26 is provided which may be switched between the contacts 26b and 26a to include either the section. dot the secondary winding It or all ofv this winding in the output circuit of the system.

To consider the resonant circuit in somewhat greater detail, it is noted that the constants of the condenser l2, the impedance of the winding of the relay 2|, the effective impedance of the transformer winding l5, and theshuntiinpedance through the reactor ll andfithe-source' ID are. soproportloned that this circuitwill resonate atva frequency which is substantially an odd integral fraction of the supply current frequency. In other words, the constants of the resonant circuit are so selected that the. supply current frequency is an odd harmonic of the resonant frequency of the circuit. In the illus-- trated arrangement, wherein a load requiring twenty cycle current istd be supplied, .a source l having-a sixty cycle outputfrequency may be used, and the constants ofthis circuit-areso chosen that the circuit will resonate at the demended. load current frequency of twenty cycles.

Inconsidering the operation of thesystem as thus far described, it will be understood that if the switch is closed, sixty cycle current is supplied from the source It! to the resonant circuit comprising the winding IS, the winding of the relay Zl and the condenser I! through the winding ofthe saturable core reactor II. It now the resonant circuit is shock-excited in the manner explained below, it will start to resonate at its fundamental frequency of twenty cycles. The resulting twenty cycle current flowing through the winding of the reactor I I, combines vectorially with the sixty cycle current flowing through this winding, so that once during each cycle of the sixty cycle wave the current traversing this winding momentarily exceeds-by a substantial amount the current value required to saturate the reactor core. During each excursion of the current beyond the knee of the saturation curve of the reactor, the reactance of the reactor drops sharply to a very low value, with the result that the current peaks sharply at a value which is several times greater than the instantaneous current values prevailing in the reactor winding during the intervals separating the peaks. These sharp transient peaks furnish the periodic shock excitation required to maintain the resonant circuit oscillating at its fundamental frequency.

The voltage which is induced in the secondary winding ll of the transformer II through the coupling between this winding and the winding II, is characterized by the fundamental and harmonic components of the twenty cycle current traversing the resonant circuit. This voltage is impressed across the twenty cycle load I8 either through the contact 26a or the contact 26b of the switch 25. It will also be understood that such high frequency harmonic components of the induced voltage as appear in the secondary winding Id of the transformer l3 are passed through the low frequency blocking condenser 22 and are impressed across the high frequency load is.

Various methods of supplying or starting a flow of current of the desired fundamental frequency in the resonant circuit may be utilized. In the usual arrangement, a large transient voltage is produced in the resonant circuit which, if of sufllcient magnitude, will cause a current of the fundamental frequency to start flowing in the resonant circuit. This current will then be sustained by the action of the non-linear impedance element or saturable core reactor ll acting in conjunction with the other elements of the circuit in the manner explained above. In conventional starting arrangements, the magnitude of the starting transient is largely determined by the tuning of the resonant circuit. For example, if a large condenser I2 is employed, then a large charge may be stored on this condenser. Alternatively, if a reactor H having a large inductance is employed, considerable energy may be stored in the magnetic field thereof. If, however, these elements are small, the energy storage capacities thereof are limited. Moreover, since it is the discharge of the stored energy which produces the starting transient and largely determines its magnitude, it will be apparent that the ease of starting is necessarily controlled to a certain extent by the tuning of the circuit. The most desirable tuning for easy starting may not be the most desirable tuning of the circuit for sustained operation thereof after an oscillatory condition of the circuit is established.

In'the present system, a starting arrangement is employed wherein the relay 2| connects the current source It) through its break contacts Ila directly to the tap l6 along the winding I! of the transformer It. With this arrangement, the condenser l2 is charged through the winding of the relay 2|. This relay is designed to be slightly slow to operate so that it remains in its restored position until the condenser 12 becomes fully charged. Upon operating, the relay 2| opens its break contacts 2 la to interrupt the low impedance path shunting the winding of the reactor ll, thus permitting the energy stored in the condenser 12 to be discharged through this winding. This discharge is of an oscillatory type and has the same frequency as the fundamental frequency of the resonant circuit. Thus a current of the resonant frequency is started flowing in the resonant circuit, which is sustained in the manner explained above.

From the above explanation, it will be understood that the voltage to which the condenser I2 is charged during a starting operation maybe readily adjusted by changing the position of the tap l6 along the winding l5, and thus altering the voltage applied to the condenser during the starting interval.

tap l6 along the winding II in no way'aflects the tuning or operation of the circuit after a sus tained oscillatory condition of the circuit is es- Changing the position of the tablished. Thus the, was voltage applied .to

the condenser I! is'controlledl'aolcly bytheposinon of the tap ll. alone the winding. is, while the tuning of the (aircdxtditrth is ended, and withthe'rela operated. is" con trolled solely bytheSposit nofl theftap'll along I the winding l5-Lj lnthisregard'it-will beund e'rj; stood that, by varylngithejposition of th tap l'l I} I ei startlnsperiod,

independently of the coupling between said cir- 2, In ,a i'requency reducing system which is gada'pted to be energigedgby. a source .of Y alternatalong the winding. lS,"theffundarnental'frequency of the resonant circuit may; hefadjusted-to the exact desired value ofTtwenty c'fyclesga accordance with the present" invention, a two winding isaturable corereactorffl-l '.-'1 i 1is-reactor includes apri'mary winding II, the terrnlnalsiof. whichmaylbe connected directly'topthe terminals of the current supply source "through the contact 30b off a two-position switch 30; or maybe bridged acro'ssthe terminals'ofthe supply source? to through a'current limiting resistor 3| and the second contact act the switch 30. This reacg tor is provided 'withasecoudary' winding 29 which is serially incIudedin thefoutputxcircuitof the system. The physical and windingxcons'tants of the reactor 21 are such thatduring each half cycle of the current supplied to the. primary wind;- ing 28 from -the scurce "Lthe coreyof 'the reactor becomes cversaturated with a. resulting distortion of-theyoitage induced in the secondary winds id o rce. an output circuit coupled'to said oscillatory circuit, ,mea'nsfor generating ,har-

Y current frequency-are produced .inthe seconvslary;

winding}! by'rtheeigcurslons of the primary current beyondqthej. saturation. bend of the reactor core saturationlcurve. Since the winding 29v is connected in series with the secondary winding I4 j ing-current .oi 'given;frequenci7. anoscillatory cir- I [cult capable 'of free oscillation atja frefquency o r than a i n frequency wh a ril ed by aid-so ce: an. o tn e m t c upl d rt 'i oscillatory circuit; means for generating hatmonics of said given frequency, "and means for Iintroducings'aid rt emes. into sald oll tl ll li on- 7 edit-indhehdentlybf the coupling' between said; ;'output and oscillatory circuits said given frequency, j ja'nd means "comprising an impedance element connected to said joutputcir cult, and coupled to said harmonic generating means forintroducjingsaid harmonicsLinto-said output circuit independently of the c'oupl'ing'between said output and" oscillatory circuits.

4. In a frequency reducing system which is adapted to be energized by a sourceof'alternat- -ing current of given frequency, an oscillatory circult capable of free oscillation at a frequency lower than said" given frequency when'energized by monies of said'gi-ven frequency, and means comof the transformer l3, it will be understood that a the voltage appearing thereacross combines vectorially with that across the winding H to be impressed across the two load circuits l8 and I9.

It will also be apparent that the turn ratio between theprimary winding 28 and the secondary winding" may be selected to introduce harmonic voltages of the required magnitude into the out The current limiting. resistor 3|, when required; is chosen 'olf' a value to prevent excessive current flow through the winding ,1! i when the system is designed to operate at relaput circuit.

tively high supply source voltages.

Among the advantages of the above-described arrangement for augmenting the high frequency components of the generated output/voltage, is that of producing in this voltage a greater number of high frequency components than are produced in the output voltages of the known systems of the prior art, wherein tone coils are used. A second advantage is that of appreciably increasing the low frequency output capabilities of the system by reducing the losses of the resonant circuit.

While one embodiment of the invention has been disclosed, it will-be understood that various modifications may be made'therein which are within the true splritand scope of the invention.

What is claimed is:

1. In a frequency reducing system which is adapted to be energized by a source of alternating current of given frequen y, a static circuit including means for converting current derived from said source into current of a lower fre-' prising an "impedance; element serially included in said outputcircuit and coupled to' said harmonic generating 'means for introducing said harmonics into said output circuit independently of. the coupling between said output and oscillatory circuits.

5. In a frequencyreducing. system which is adapted to be energized by a. source'of alternating current of given frequency,=a resonant circult tuned to oscillate freelyat a fundamental frequency which is lower than said given frequency, means including a non-linear impedance e-lement for'energizing said circuit from said alternating current source and for causing; said -"circuitto oscillate at itsfundamental frequency, I

an output circuit, a-secondnon-linear imped-. 1 ance element connected 'to said source and adapted to: produce harmonics-oi said given frequency when energizedfrom said source, an output oircult, and means for introducing the harmonics generated by said second impedance element into said. output circuit.

6.'In a frequency reducing system which is 77 adapted to'be energized by a source of alternating current of given frequency, a resonant cirv cuit tuned to oscillate freely at a fundamental frequency which is lower than said given frev quency, means including a non-linear impedance 4 element for energizing said circuit from said alternating current source and for causing said cirnamed circuit, means for generating harmonics of said given frequency, and means for introducing said harmonics into said output circuit cult to oscillate at its fundamental frequency,- an output circuit coupled to said resonant circuit, a second non-linear impedance element con nected to said source and adapted to produce harmonics of said given frequency when energized by said source, and means for-introducing the harmonics generated by said second impedance element into said output circuit independently of the coupling between said circuits.

7. In a frequency reducing system which is adapted to be energized by a source of alternat- 3. In aqfrequencyl-reduclng systemsyvhich is. adapted to be energizedbya source-of alternating current of given frequency ,--an oscillatory circuit capable ofv free oscillati'onat a frequency'lowen (than. said given frequency when energized by said source; an outputfcircuit coupled to" said oscillatorycircuit; means; or; generating harmonics of ing current of given frequency, a resonant circuit tuned to oscillate freely at a fundamental frequency which is lower than said given fre-- quency, means including a non-linear impedance element for energizing said circuit from said alternating current source and for causing said circuit to oscillate at its fundamental frequency, an output circuit coupled to said resonant circuit, a second non-linear impedance element connected to said source and adapted to produce harmonics of said given frequency when energized by said source, and means including an impedance element serially included in said output circuit and coupled to said second impedance element for introducing the harmonics generated by said second impedance element into said output circuit independently of the coupling between said circuits.

8. In a frequency reducing system which is adapted to be energized by a source of alternating current of given frequency, a resonant cir cuit tuned to oscillate freely at a fundamental frequency which is lower than said given frequency, means including a non-linear impedance element for energizing said circuit from said alternating current source and for causing said circuit to oscillate at its fundamental frequency, an output circuit, a saturable core reactor including a pair of windings and operative to generate harmonics of said given frequency in one of said windings when the other of said windings is energized by said source, means connecting said other winding to be energized by said source, and means for introducing the harmonics generated in said one winding into said output circuit.

9. In a frequency reducing system which is adapted to be energized by a source of alternating current of given frequency, a resonant circuit tuned to oscillate freely at a fundamental frequency which is lower than said given frequency, means including a non-linear impedance element for energizing said circuit from said alternating current source and for causing said circuit to oscillate at its fundamental frequency, an output circuit coupled to said resonant circuit, a saturable core reactor including a pair of windings and operative to generate harmonics of said given frequency in one of said windings when the other of said windings is connected to be energized by said source, means connecting the other of said windings to be energized by said source, and means independent of the coupling between said output and resonant circuits for introducing the harmonics generated in said one winding into said output circuit.

10. In a frequency reducing system which is adapted to be energized by a source of alternatlng current of given frequency, a resonant circuit tuned to oscillate freely at a fundamental frequency which is lower than said given frequency, means including a non-linear impedance element for energizing said circuit from said alternating current source and for causing said circuit to oscillate at its fundamental frequency, an output circuit, a saturable core reactor including a pair of windings and operative to generate harmonies of said given frequency in one of said windings when the other of said windings is energized by said source, means connecting said other winding to be energized by said source, and means serially connecting said one winding in said output circuit.

11. In a frequency reducing system which is adapted to be energized by a source of alternating current of given frequency, a resonant circuit tuned to oscillate freely at a fundamental frequency which is lower than said given frequency, means including a non-linear impedance element for energizing said circuit from said alternating current source and for causing said circuit to oscillate at its fundamental frequency, an output circuit, a saturable core reactor including a pair of windings and operative to generate harmonics of said given frequency in one of said windings when the other of said windings is energized by said source, means comprising a current limiting resistor connected in series with said other winding for connecting said other winding to be energized by said source, and means for introducing the harmonics generated in said one winding into said output circuit.

12. In a frequency reducing system which is adapted to be energized by a source 0f alternating current of given frequency, a resonant circuit tuned to oscillate freely at a fundamental frequency which is lower than said given frequency, means including a non-linear impedance element for energizing said circuit from said alternating current source and for causing said circuit to oscillate at its fundamental frequency, an output circuit coupled to said resonant circuit, a saturable core reactor including a pair of windings and operative to generate harmonics of said given frequency in one of said windings when the other of said windings is connected to be energized by said source, means comprising a current limiting resistor connected in series with said other winding for connecting the other of said windings to be energized by said source, and means independent of the coupling between said circuits for introducing the harmonics generated in said one winding into said output circuit.

ROSWELL H. HERRICK. 

